Well, I've also done some thought experiment on this, and looked at it from the viewpoint of your average M.E. Islamist/AlQuida recruit.
Imagine you
hate China, for all sorts of reasons. And you've joined up with an organization that wants to put some serious hurt on them, and you're willing to die in the process.
So how "easy" is it for you, an American to emigrate to China, get a job, get imbedded in their society, and then after a time, either through the black market, or your own network procure weapons and explosives, go shoot up a school or whatever over there.
Seems kind of crazy and daunting, no?
Now granted American society is much more open and people of all nationalities can "blend" in much better than any non-Chinese could in China. However, THEY don't know this in their gut. Coming to America is just as daunting a challenge as it would be for any of us picked at random to go to China.
Now, instead, imagine there's Chinese troops just over the border in Canada, and you live in Minnesota, and all you need to do is drive over the border with some guns to try and kill a few of them...
That's why despite all our provocation, there's been so much activity over in Iraq and Afghanistan, but so little here, despite how "easy" it seems.
It takes time to build a network of disgruntled Americans who are otherwise as comfortable as we are moving about in our society before they strike.